The Milky Way Tomography With Sdss. Iii. Stellar Kinematics

Abstract

We study Milky Way kinematics using a sample of 18.8 million main-sequence stars with r < 20 and proper-motion measurements derived from SDSS and POSS astrometry, including ∼170,000 stars with radial-velocity measurements from the SDSS spectroscopic survey. Distances to stars are determined using a photometric parallax relation, covering a distance range from ∼100 pc to 10 kpc over a quarter of the sky at high Galactic latitudes (|b| > 20 • ). We find that in the region defined by 1 kpc < Z < 5 kpc and 3 kpc < R < 13 kpc, the rotational velocity for disk stars smoothly decreases, and all three components of the velocity dispersion increase, with distance from the Galactic plane. In contrast, the velocity ellipsoid for halo stars is aligned with a spherical coordinate system and appears to be spatially invariant within the probed volume. The velocity distribution of nearby (Z < 1 kpc) K/M stars is complex, and cannot be described by a standard Schwarzschild ellipsoid. For stars in a distance-limited subsample of stars (<100 pc), we detect a multimodal velocity distribution consistent with that seen by HIPPARCOS. This strong non-Gaussianity significantly affects the measurements of the velocity ellipsoid tilt and vertex deviation when using the Schwarzschild approximation. We develop and test a simple descriptive model for the overall kinematic behavior that captures these features over most of the probed volume, and can be used to search for substructure in kinematic and metallicity space. We use this model to predict further improvements in kinematic mapping of the Galaxy expected from Gaia and LSST.

Isotropic at the Break? 3d Kinematics of Milky Way Halo Stars in the Foreground of M31

“…Local studies, limited to heliocentric distances D10 kpc, have measured the full 3D kinematics of halo stars. This has revealed a strongly radially biased velocity anisotropy with 1 0.5 0.7 tan 2 rad 2 -b s s = -» (e.g., Smith et al 2009;Bond et al 2010), in seemingly good agreement with the predictions of simulations (e.g., Bullock & Johnston 2005;Cooper et al 2010).…”

“…Our measurement of β, from the 3D kinematics of N=13 stars in the radial range 18r/kpc32, is shown in pink. Solar neighborhood measurements, using full 3D velocity information, find a radially biased β, shown in red Bond et al 2010). The remaining error bars show estimates of β for distant (D10 kpc) halo stars using LOS velocity distributions (Sirko et al 2004;Deason et al 2012;Kafle et al 2012;King et al 2015).…”

Local Group Proper Motion Dynamics

“…They measured the PMs of 13 stars at 24 ± 6 kpc toward the Andromeda-Triangulum halo overdensity, which imply a halo velocity ellipsoid that is roughly isotropic. This is more tangential than the velocity ellipsoid of halo stars near the Sun, which is measured from ground-based PMs to be highly radially anisotropic (e.g., Bond et al 2010). These results are consistent with the hypothesis that the Andromeda-Triangulum overdensity may be a shell in the halo, possibly resulting from an ancient accretion event.…”

The Milky Way's rotation curve out to 100 kpc and its constraint on the Galactic mass distribution

“…For halo tracers, profile of the radial velocity dispersion σr can be easily estimated from the line-of-sight velocity dispersion σ los (Battaglia et al 2005;Dehnen et al 2006), while their number density is found to follow a double power law with a break radius r b around 20 kpc (Bell et al 2008;Watkins et al 2009;Deason et al 2011;Sesar et al 2011). However, the anisotropy parameter β has only been accurately measured in the solar neighborhood, with a radially biased value between 0.5 -0.7 (Kepley et al 2007;Smith et al 2009;Bond et al 2010;Brown et al 2010). Due to the lack of accurate proper motion measurements of distant halo tracers, the anisotropy parameter β is still poorly constrained beyond the solar neighborhood, particularly for the outer halo (> 25 kpc).…”